This invention relates to a method of manufacturing a reference voltage generator.
In generating reference voltages in various semiconductor electronic circuits, it is an indispensable condition to utilize any physical quantity having the dimension of the voltage. As the physical quantities, there have heretofore been utilized such quantities as the forward voltage drop V.sub.F or reverse breakdown voltage (Zener voltage) V.sub.Z of a PN-junction diode, the threshold voltage V.sub.th of an insulated gate field-effect transistor (often represented by an IGFET or MOSFET), etc.
These physical quantities do not indicate voltage values which are absolutely fixed. On the contrary, their voltage values are subject to fluctuations due to various factors. In order to make use of the physical quantities for the reference voltage generator devices of various electronic circuits, accordingly, attention must be paid to factors of the fluctuations of the voltage values to be obtained and allowable widths of the fluctuations.
First of all, as to the temperature characteristics of the physical quantities, the voltages V.sub.F and V.sub.th usually have a temperature-dependency of approximately 2-3 mV/.degree.C. The fluctuation of the reference voltage attendant upon the temperature change attains such a magnitude that the application has to be given up in some uses.
By way of example, when a battery checker for giving the alarm that the voltage of a battery has lowered below a predetermined reference value is intended to be realized in an electronic timepiece which employs a silver oxide battery having a nominal voltage of 1.5 V, whether the battery voltage is high or low needs to be judged with the boundary (detection level) or the detection reference value at about 1.4 V.
When a reference voltage generator device is to be constructed by exploiting the threshold voltage V.sub.th of a MOSFET or the forward drop voltage V.sub.F of a diode which is about 0.6 V, the detection level aimed at 1.4 V has a temperature-dependency of: ##EQU1##
Accordingly, even when the practical operating temperature range is estimated as narrow as 0.degree. C. to 50.degree. C., the detection level fluctuates as greatly as 1.23 V to 1.57 V, and a practical battery checker cannot be obtained.
Subsequently, the physical quantities have dispersions or deviations in the manufacture. For example, their threshold voltage V.sub.th of a MOSFET has a dispersion of about .+-.0.2 V, which is greater than the temperature fluctuation. Accordingly, in a case where the above-stated battery checker is put into the form of an IC (integrated circuit) by exploiting the voltage V.sub.th, not only external components and external connection pins (external connection terminals) for adjusting the reference voltage but also the labor of the adjustment after the fabrication of the IC is required.
The lower voltage limit of the Zener voltage V.sub.z is about 3 V, and it is impossible to generate a reference voltage to be used in a low voltage range of 1 to 3 V or so. In a case of using the Zener voltage or the forward drop voltage of a diode as a reference voltage, a current on the order of several mA to several tens of mA needs to be caused to flow, which is inappropriate to the point of lowering the power dissipation of the reference voltage generator device.
As is apparent from the above explanation, the conventional reference voltage generator devices exploiting the voltages V.sub.th, V.sub.F and V.sub.z have not always been suited to all the uses when the temperature characteristics, the dispersions or deviations in manufacture, the power dissipation, the voltage level etc. have been taken into account. For uses requiring very severe charactetistics, it has been often the case that the practical use or the mass production must be relinquished.
The inventors have from their studies as above described that the improvements in the conventional reference voltage generator devices are subject to limitations physically, and have therefore developed a reference voltage generator device which has a new idea and concept.